Pyrogenic silicon dioxide powder and dispersion thereof

ABSTRACT

Pyrogenically produced silicon dioxide powder with a specific surface area of between 5 and 600 m 2 /g and a carbon content of less than 500 ppm, which displays a specific dibutyl phthalate absorption of less than or equal to 1.2 g dibutyl phthalate/100 g SiO2 per m 2  of specific surface area and a specific thickening action of less than 15 mPas/m 2  of specific surface area. It is produced by supplying vaporous tetramethoxysilane and/or tetraethoxysilane together with air and separately hydrogen to a burner, and allowing the mixture of gases to react in a flame in a reaction chamber connected in series to the burner, and separating the solid reaction product from the gas stream by known means, the lambda value in the burner being between 0.95 and 1.5 and sufficient secondary air also being supplied to the reaction chamber that the lambda value in the reaction chamber is between 0.8 and 1.6. The invention also provides a dispersion containing the silicon dioxide powder and the use of the powder and of the dispersion.

The invention provides a pyrogenically produced silicon dioxide powder,an aqueous dispersion containing this silicon dioxide powder, theproduction and use of the silicon dioxide powder and the dispersion.

The term pyrogenic silicon dioxide or pyrogenic silica (fumed silica) isa collective term for all highly disperse silicas obtained in the gasphase at elevated temperatures by coagulation of monomeric silica. Thereare two processes for the industrial production of pyrogenic silicas,high-temperature hydrolysis and the arc process.

In the high-temperature hydrolysis process a homogeneous mixture of avaporous silicon compound, hydrogen, oxygen and an inert gas is burnedwith a burner in a cooled combustion space. Two reactions proceed sideby side here. Firstly the reaction of hydrogen and oxygen with formationof water and secondly the hydrolysis of the silicon compound withformation of silicon dioxide.

The homogeneity of the gas mixture means that the reaction conditionsand hence the formation and growth conditions are largely the same foreach SiO₂ particle, such that very homogeneous and uniform particles canform. Air is used as the oxygen source in the known process. Thepyrogenic silicas produced by the known process display specific surfaceareas of between 10 and 600 m²/g.

The starting material for the silicon dioxide is generally silicontetrachloride (cf. Ullmann's Encyclopedia of Industrial Chemistry, Vol.A23, pages 635 ff 5^(th) edition). In addition to silicon tetrachloridemethyl trichlorosilane, trichlorosilane or mixtures thereof with silicontetrachloride can also be used.

JP 2002114510 claims a process in which silicon dioxide is obtained withan average particle size of 0.05 to 5 μm. In this process siliconcompounds are burned in the presence of oxygen and hydrogen. Siloxanes,silanes or silicon chlorides can be used as the silicon compound.However, the silicon dioxide produced by this process displays noproperties that could not also be obtained by processes of the priorart. The process described is itself only of limited suitability for theproduction of larger quantities. A non-uniform product and, wherecarbon-containing silicon compounds are used as starting materials, darkproducts too are then to be expected in particular.

When used in dispersions such as are used in the production of glassarticles or in chemical mechanical polishing in the semiconductorindustry, the powder produced according to JP 2002114510 provides noadvantages over the prior art.

Due to growing requirements an improvement in the properties of silicondioxide is demanded in these very sectors. In the glass industry inparticular, highly filled, readily manageable dispersions, in otherwords ones with low viscosity, are required because of their lowshrinkage on drying and sintering.

The object of the invention is to provide a silicon dioxide powder whichis suitable for the production of highly filled dispersions with lowviscosity. The object of the invention is also to provide a stabledispersion containing this silicon-dioxide powder.

The invention provides a pyrogenically produced silicon dioxide powderhaving a specific surface area of between 5 and 600 m²/g and a carboncontent of less than 500 ppm, which is characterised in that it displays

-   -   a specific dibutyl phthalate absorption of less than or equal to        1.2 g dibutyl phthalate/100 g SiO₂ per m² of specific surface        area    -   and a specific thickening action of less than 15 mPas per m² of        specific surface area.

The specific dibutyl phthalate absorption represents a measure of thestructure of the silicon dioxide powder according to the invention as afunction of its specific surface area. The term structure in thisconnection means the degree of intergrowth of the primary particles.These are initially formed in the pyrogenic process and as the reactioncontinues can coalesce to form chain-like aggregates, which in turn formagglomerates. The specific dibutyl phthalate absorption of less than orequal to 1.2 g dibutyl phthalate/100 g SiO₂ per m² of specific surfacearea claimed for the silicon dioxide powder according to the inventionis generally lower than pyrogenic silicon dioxide powders obtained bythe prior art.

The silicon dioxide powder according to the invention arises only incombination with a specific thickening action. This is understood tomean the thickening action per m² of specific surface area. Thethickening action is determined in a dispersion of a silicon dioxidepowder in a polyester.

In a preferred embodiment the powders according to the invention candisplay a specific compacted bulk density, defined as the product of thecompacted bulk density and specific surface area, of between 1000 and10000 and particularly preferably between 4000 and 7000 g/l×m² ofspecific surface area. Powders according to the invention displaying aspecific compacted bulk density in this range can be incorporatedespecially readily into dispersions.

Furthermore, silicon dioxide powders according to the invention canpreferably have a chloride content of less than 50 ppm, particularlypreferably less than 20 ppm. The low chloride contents can for exampledemonstrate advantageous effects when the powders according to theinvention are used in the area of chemical mechanical polishing.

The invention also provides a process for the production of silicondioxide powder according to the invention which is characterised in that

-   -   vaporous tetramethoxysilane (TMOS) and/or tetraethoxysilane        (TEOS) together with air or with oxygen-enriched air and        separately    -   hydrogen    -   are supplied to a burner, and the mixture of gases is allowed to        react in a flame in a reaction chamber connected in series to        the burner, and the solid reaction product is separated from the        gas stream by known means,    -   the lambda value in the burner being between 0.95 and 1.5 and    -   sufficient secondary air also being supplied to the reaction        chamber that the lambda value in the reaction chamber is between        0.8 and 1.6.

FIG. 1 shows a simplified process flow chart upon which the processaccording to the invention is based. A=burner; B=flame; C=reactionchamber;

1=supply of mixture comprising vaporous tetramethoxysilane and/ortetraethoxysilane together with air or with oxygen-enriched air;2=supply of hydrogen; 3=supply of secondary air.

In the performance of the process it is substantial that a premixing ofsilane and air occurs, the stoichiometry of air/hydrogen and the oxygencomponent, expressed as the lambda value, being maintained in the burnerand reaction chamber.

Lambda denotes the ratio of oxygen supplied to the burner or thereaction chamber to stoichiometrically required oxygen, which is neededto convert the silane compound completely to silicon dioxide. The lambdavalue range that must be maintained in the reaction chamber likewiserefers to the total amount of the silane to be hydrolysed.

In the process according to the invention the volume ratio ofoxygen/hydrogen in the burner can be varied between 0.2 and 2.8. In aparticularly preferred embodiment the volume ratio of oxygen/hydrogen inthe burner is between 0.9 and 1.4.

Depending on the desired specific surface area, it can be useful to varythe streams supplied to the burner and the burner geometry in such a waythat the discharge velocity of the gases leaving the burner is at least10 ms⁻¹. Discharge velocities of at least 20 ms⁻¹ are particularlypreferred.

The invention also provides an aqueous dispersion containing the silicondioxide powder according to the invention.

The aqueous dispersion according to the invention can display a contentof silicon dioxide powder of between 20 and 80 wt. %. Dispersions havinga content of silicon dioxide powder of between 40 and 60 can beparticularly preferred. These dispersions display a particularly highstability with a comparatively low structure.

The aqueous dispersion according to the invention can preferably displayan average particle size in the aggregates of silicon dioxide powderwhich is less than 200 nm. For certain applications such as e.g. thechemical mechanical polishing of semiconductor substrates, a value ofless than 150 nm can be particularly preferred.

The dispersion according to the invention can be stabilised by theaddition of bases or cationic polymers or aluminium salts or a mixtureof cationic polymers and aluminium salts or acids.

Bases that can be used are ammonia, ammonium hydroxide, tetramethylammonium hydroxide, primary, secondary or tertiary organic amines,sodium hydroxide solution or potassium hydroxide solution.

Cationic polymers that can be used are examples having at least onequaternary ammonium group, phosphonium group, an acid adduct of aprimary, secondary or tertiary amine group, polyethylene imines,polydiallylamines or polyallylamines, polyvinylamines, dicyandiamidecondensates, dicyandiamide-polyamine cocondensates orpolyamide-formaldehyde condensates.

Aluminium salts that can be used are aluminium chloride, aluminiumhydroxychlorides having the general formula Al(OH)_(x)Cl where x=2-8,aluminium chlorate, aluminium sulfate, aluminium nitrate, aluminiumhydroxynitrates having the general formula Al(OH)_(x)NO₃ where x=2-8,aluminium acetate, alums such as aluminium potassium sulfate oraluminium ammonium sulfate, aluminium formate, aluminium lactate,aluminium oxide, aluminium hydroxide acetate, aluminium isopropylate,aluminium hydroxide, aluminium silicates and mixtures of theaforementioned compounds.

Inorganic acids, organic acids or mixtures of the aforementioned can beused as acids.

In particular, phosphoric acid, phosphorous acid, nitric acid; sulfuricacid, mixtures thereof and their acid-reacting salts can be used asinorganic acids.

Organic acids that are preferably used are carboxylic acids having thegeneral formula C_(n)H_(2n+1)CO₂H, where n=0-6 or n=8, 10, 12, 14, 16,or dicarboxylic acids having the general formula HO₂C(CH₂)_(n)CO₂H,where n=0-4, or hydroxycarboxylic acids having the general formulaR₁R₂C(OH)CO₂H, where R₁=H, R₂=CH₃, CH₂CO₂H, CH(OH)CO₂H, or phthalic acidor salicylic acid, or acid-reacting salts of the aforementioned acids ormixtures of the aforementioned acids and salts thereof.

Stabilisation of the dispersion according to the invention withtetramethyl ammonium hydroxide or aluminium hydroxychloride in an acidmedium can be particularly advantageous.

The dispersion can optionally also contain other additives. These canfor example be oxidising agents such as hydrogen peroxide or per-acids,oxidation activators whose purpose is to increase the rate of oxidation,corrosion inhibitors such as e.g. benzotriazole. Surface-activesubstances of a non-ionic, cationic, anionic or amphoteric nature canalso be added to the dispersion according to the invention.

The invention also provides a process for the production of thedispersion according to the invention, which is characterised in thatthe silicon dioxide powder according to the invention is incorporatedwith a dispersing device into water, which can be stabilised by theaddition of bases or cationic polymers or aluminium salts or a mixtureof cationic polymers and aluminium salts or acids, and is then dispersedfurther for a period of 5 to 30 minutes.

There is no restriction on the type of dispersing device. It can beadvantageous however, especially for the production of highly filleddispersions, to use dispersing devices with a high energy input. Thesecan for example be rotor-stator systems, planetary compounders orhigh-energy mills. In the latter, two predispersed streams of suspensionunder high pressure are decompressed through a nozzle. The two jets ofdispersion hit each other exactly and the particles grind themselves. Inanother embodiment the predispersion is likewise placed under highpressure, but the particles collide against armoured sections of wall. Arotor-stator system can preferably be used to produce the dispersionaccording to the invention.

The invention also provides the use of the silicon dioxide powderaccording to the invention as a filler in rubber, silicone rubber andplastics, to adjust the rheology in paints and coatings and as a supportfor catalysts.

The invention also provides the use of the dispersion according to theinvention for the production of glass articles, for chemical mechanicalpolishing and for the production of inkjet papers.

EXAMPLES

Analytical Determinations

The specific surface area of the powders is determined in accordancewith DIN 66131.

The dibutyl phthalate absorption is measured with a RHEOCORD 90 devicesupplied by Haake, Karlsruhe. To this end 8 g of the silicon dioxidepowder is introduced into a mixing chamber with an accuracy of 0.001 g,the chamber is closed with a lid and dibutyl phthalate is metered inthrough a hole in the lid at a predefined feed rate of 0.0667 ml/s. Thecompounder is operated at a motor speed of 125 revolutions per minute.On reaching the maximum torque the compounder and DBP metering areautomatically switched off. The DBP absorption is calculated from theconsumed amount of DBP and the weighed amount of particles according tothe formula below:DBP value (g/100 g)=(DBP consumption in g/weighed amount of particles ing)×100.

The thickening action is determined by the following method: 7.5 gsilicon dioxide powder are introduced into 142.5 g of a solution of anunsaturated polyester resin in styrene with a viscosity of 1300+/−100mPas at a temperature of 22° C. and dispersed by means of a high-speedmixer at 3000 min⁻¹. A suitable example of an unsaturated polyesterresin is Ludopal® P6, BASF. A further 90 g of the unsaturated polyesterresin in styrene are added to 60 g of this dispersion and the dispersionprocess is repeated. The thickening action is taken to be the viscosityvalue in mPas of the dispersion at 25° C., measured with a rotaryviscometer at a shear rate of 2.7 s⁻¹.

The chloride content of the silicon dioxide powder is determined by thefollowing procedure: Approximately 0.3 g of the particles according tothe invention are weighed in accurately, topped up with 20 ml of 20percent reagent-grade sodium hydroxide solution, dissolved andtransferred into 15 ml cooled HNO₃ whilst being stirred. The chloridecontent in the solution is titrated with AgNO₃ solution (0.1 mol/l or0.01 mol/l).

The carbon content of the silicon dioxide powder is determined by thefollowing procedure: Approximately 100 to 1000 mg of the particlesaccording to the invention are weighed accurately into a crucible,combined with 1 g each of ultrapure iron and aggregate (LECOCELL II) andburned in a carbon analyser (LECO) at approx. 1800° C. with the aid ofoxygen. The CO₂ that is generated is measured by IR and the contentcalculated therefrom.

The compacted bulk density is determined by reference to DIN ISO 787/XIK 5101/18 (not screened).

The pH is determined by reference to DIN ISO 787/IX, ASTM D 1280, JIS K5101/24.

The viscosity of the dispersions is determined with a Physica Model 300rotary rheometer and a CC 27 measuring beaker at 25° C. The viscosityvalue is determined at a shear rate of 10 1/sec. This shear rate is in arange in which the viscosity of the dispersions formed is virtuallyindependent of the shear stress.

The particle size prevailing in the dispersion is determined by means ofdynamic light scattering. A Zetasizer 3000 HSa (Malvern Instruments, UK)is used. The volume-weighted median value of the peak analysis isstated.

Example 1

1.5 kg/h tetramethoxysilane are evaporated at 180° C. and introducedinto the central pipe of the burner. 12 m³/h of air are additionallyintroduced into the central pipe. 1.8 m³/h of hydrogen are fed into apipe surrounding the central pipe. The gas mixture burns in the reactionchamber, into which 17 m³/h of secondary air are additionallyintroduced.

The reaction gases and the silicon dioxide that is formed are drawnthrough a cooling system by application of a partial vacuum, coolingthem to values between 100 and 160° C. The solid is separated from thewaste gas stream in a filter or cyclone.

The analytical data for the silicon dioxide powder obtained isreproduced in Table 2.

Examples 2 to 9 and comparative examples 10 and 11 were performed in thesame way.

In comparative examples 12 to 14 silicon tetrachloride is used in placeof tetramethoxysilane. In these experiments, following separation fromthe waste gas stream the silicon dioxide powder is treated at elevatedtemperature with water vapour-containing air to remove adheringhydrochloric acid residues.

The physical-chemical data for the silicon dioxide powders obtained isreproduced in Table 2.

Examples 1 to 9 lead to the silicon dioxide powders according to theinvention having a low structure, expressed as the specific DBP value, alow specific thickening action and a high specific compacted bulkdensity.

Examples 10 and 11 show that only the process according to the inventionleads to these powders. Reducing the secondary air or even omitting italtogether or increasing the burner air does not lead to the silicondioxide powders according to the invention.

In the same way, using silicon tetrachloride, examples 12 to 14, whilstmaintaining the conditions with regard to the lambda value in the burnerand in the reaction chamber, does not lead to the silicon dioxidepowders according to the invention.

Example 15 Production of a Dispersion in the Acid pH Range

36 kg of demineralised water are placed in a 60 l stainless steel batchcontainer. 6.4 kg of the pyrogenically produced silicon dioxide are thendrawn in under shear conditions using the suction pipe of the YstralConti-TDS 3 and on completion of the drawing-in process shearing iscontinued for a further 15 min at 3000 rpm.

Example 16 Production of a Dispersion in the Alkaline pH Range

35.5 kg of demineralised water and 52 g of a 30% KOH solution are placedin a 60 l stainless steel batch container. 6.4 kg of the pyrogenicallyproduced silicon dioxide are then drawn in under shear conditions usingthe suction pipe of the Ystral-Conti-TDS 3 and on completion of thedrawing-in process shearing is continued for a further 15 min at 3000rpm. During this 15-minute dispersion the pH is adjusted to and held ata pH of 10.4 by addition of further KOH solution. A further 43 g of KOHsolution were used in this process and a solids concentration of 15 wt.% established by addition of 0.4 kg water.

Example 17 Production of a Dispersion in the Presence of Aluminium Salts

35 kg of demineralised water are placed in a 60 l stainless steel batchcontainer. 6.4 kg of the pyrogenically produced silicon dioxide are thendrawn in under shear conditions using the suction pipe of the YstralConti-TDS 3. 640 g of a 1 wt. % solution (relative to aluminium oxide)of aluminium chloride are then added with dispersion and on completionof the addition shearing is continued for a further 15 min at 3000 rpm.0.1 kg demineralised water and 305 g 1 N NaOH are then added to obtain a15 wt. % dispersion with a pH of 3.5.

Example 18 Production of a Dispersion of Aerosil 90 (ComparativeExample)

35.5 kg of demineralised water and 52 g of a 30% KOH solution are placedin a 60 l stainless steel batch container. 5.2 kg of AEROSIL® 90 arethen drawn in under shear conditions using the suction pipe of theYstral Conti-TDS 3 and on completion of the drawing-in process shearingis continued for a further 15 min at 3000 rpm. During this 15-minutedispersion the pH is adjusted to and held at a pH of 10.4 by addition offurther KOH solution. A further 63 g of KOH solution were used in thisprocess and a solids concentration of 15 wt. % established by additionof 0.6 kg water.

The physical-chemical parameters for the dispersions are reproduced inTable 3. TABLE 3 Physical-chemical data for the dispersions Averageparticle Viscosity Concentration size (10 s⁻¹) Ex. SiO₂ [wt. %] pH [nm][mPas] 15 From ex. 7 15 3.7 101 4.1 16 From ex. 7 15 10.4 103 1.9 17From ex. 7 15 3.5 107 2.4 18 Aerosil 90* 12.5 10.4 198 3.5*Pyrogenically produced silicon dioxide from Degussa AG, BET surfacearea approx. 90 m²/g.

Example 19 Dispersion with High Solids Content

35.5 kg of demineralised water in a 60 l stainless steel batch containerare adjusted to a pH of 11 with tetramethyl ammonium hydroxide solution(25%). 37 kg of the pyrogenically produced silicon dioxide are thendrawn in under shear conditions using the suction pipe of the YstralConti-TDS 3 and on completion of the drawing-in process shearing iscontinued for a further 15 min at 3000 rpm. During this 15-minutedispersion the pH is held at a pH of between 10 and 11 by addition oftetramethyl ammonium hydroxide solution. A solids concentration of 50wt. % is established by addition of the remaining amount of water thatis needed.

The resulting dispersion has a silicon dioxide content of 50 wt. % and apH of 10.3. It displays a viscosity, determined with a Physicaviscometer, of 2450 mPas. The average particle size is 116 nm. Thedispersion displays no thickening or sedimentation even after a storageperiod of 6 months.

The silicon dioxide powders according to the invention are characterisedby an ability to be incorporated rapidly into aqueous media.

In comparison to dispersions with the known silicon dioxide powder, thedispersions according to the invention display more favourable valuesfor viscosity and smaller particle sizes.

Example 19 shows that dispersions with a high solids content can also beproduced. Under similar conditions the use of known silicon dioxidepowders with a comparable BET surface area leads to gel-likecompositions, or the powder cannot be incorporated fully. TABLE 1Amounts used and settings from examples 1 to 14 H₂ Air Secondary LambdaTMOS burner burner air Lambda reaction O₂/H₂ v* [kg/h] [m³/h] [m³/h][m³/h] burner chamber burner [m/s] Example 1 1.5 1.8 12 17 1.11 2.32 1.430.0 2 1.5 2.3 12 17 1.01 2.09 1.1 31.0 3 1.5 3.4 14.8 17 1.02 1.96 0.939.1 4 1.5 2.4 14.8 17 1.19 2.27 1.3 37.0 5 1.5 3 14.8 17 1.08 2.07 1.038.2 6 1.5 2.4 12 17 1.00 1.88 1.1 31.2 7 1.5 2.4 12 17 1.00 1.83 1.131.2 8 1.5 2.4 12 17 1.00 2.09 1.1 31.2 9 1.5 1.8 12 17 1.11 2.31 1.430.0 Comp. ex. 10  1.5 1.8 12 0 1.11 1.05 1.4 30.0 11  1.5 1.8 12 5 1.111.05 1.4 30.0  12** 4.4 2 5.8 17 1.21 3.79 0.61 17.6 13  4.4 2 5.2 171.09 3.68 0.55 16.3 14  4.4 2.3 5.5 17 1 3.30 0.50 17.6*= Discharge velocity from burner;**2 to 14: SiCl₄ in place of TMOS

TABLE 2 Physical-chemical values of the silicon dioxide powders fromexamples 1 to 14 Spec. Spec. DBP Spec. Comp. comp. DBP number thickeningbulk bulk BET number [g/100 g]/ Thickening [mPas]/ density density C Cl[m²/g] [g/100 g] [m²/g] [mPas] [m²/g] [g/l] [g/l] × [m²/g] pH μg/g μg/gExample 1 200 225 1.1 1920 9.6 24 4800 4.24 <100 <13 2 129 146 1.1 7705.9 35 4515 4.36 <100 <10 3 163 98 0.6 830 5.1 30 4890 4.38 <100 <10 4330 314 1.0 2220 6.7 19 6270 3.98 <100 <10 5 196 125 0.6 1650 8.4 265096 4.05 <100 <13 6 109 74 0.7 690 6.3 42 4578 4.53 <100 <10 7 99 1101.1 1325 13.4 50 4950 4.22 <100 <10 8 130 95 0.7 740 5.7 35 4550 4.13<100 <10 9 191 191 1.0 1590 8.3 21 4011 4.00 <100 <14 Comp. ex. 10  205320 1.6 3100 15.1  n.d.* n.d. 4.11 <100 <13 11  198 280 1.4 3050 15.4n.d. n.d. 4.09 <100 <10 12  199 346 1.7 3230 16.2 17 3383 4.09 <100 8013  131 309 2.4 1880 14.4 19 2489 4.17 <100 44 14  91 233 2.6 2805 30.824 2184 4.23 <100 83*n.d. = not determined

1. A pyrogenically produced silicon dioxide powder having a specificsurface area of between 5 and 600 m²/g and a carbon content of less than500 ppm, wherein the pyrogenically produced silicon dioxide powderdisplays a specific dibutyl phthalate absorption of less than or equalto 1.2 g dibutyl phthalate/100 g SiO₂ per m² of specific surface areaand a specific thickening effect of less than 15 mPas per m² of specificsurface area.
 2. The pyrogenically produced silicon dioxide powderaccording to claim 1, wherein the specific compacted bulk density isbetween 1000 and 10000 g/l×m² of specific surface area.
 3. Thepyrogenically produced silicon dioxide powder according to claim 1,wherein the chloride content is less than 50 ppm.
 4. A process for theproduction of the pyrogenically produced silicon dioxide powderaccording to claim 1, wherein vaporous tetramethoxysilane and/ortetraethoxysilane together with air or with oxygen-enriched air andseparately hydrogen are supplied to a burner, and the mixture of gasesis allowed to react in a flame in a reaction chamber connected in seriesto the burner, and the solid reaction product is separated from the gasstream by known means, the lambda value in the burner being between 0.95and 1.5 and sufficient secondary air also being supplied to the reactionchamber that the lambda value in the reaction chamber is between 0.8 and1.6.
 5. The process according to claim 4, wherein the volume ratio ofoxygen/hydrogen in the burner is between 0.2 and 2.8.
 6. The processaccording to claim 4, wherein the discharge velocity of the gasesleaving the burner is at least 10 ms⁻¹.
 7. An aqueous dispersioncomprising the pyrogenically produced silicon dioxide powder accordingto claim
 1. 8. The aqueous dispersion according to claim 7, wherein thecontent of silicon dioxide in the dispersion is between 20 and 80 wt. %.9. The aqueous dispersion according to claim 7, wherein the averageaggregate diameter in the dispersion is less than 200 nm.
 10. Theaqueous dispersion according to claim 7, wherein the aqueous dispersioncontains additives.
 11. A process for the production of the aqueousdispersion according to claim 7, wherein the pyrogenically producedsilicon dioxide powder is incorporated with a dispersing device intowater, which can be stabilised by the addition of bases or cationicpolymers or aluminium salts or a mixture of cationic polymers andaluminium salts or acids, and is then dispersed.
 12. Use of the silicondioxide powder according to claim 1 in dispersions, as a filler inrubber, silicone rubber and plastics, to adjust the rheology in paintsand coatings, as a support for catalysts.
 13. Use of the dispersionaccording to claim 7 for the production of glass articles, for chemicalmechanical polishing, for the production of inkjet papers.
 14. Acomposition comprising the pyrogenically produced silicon dioxide powderaccording to claim
 1. 15. The composition as claimed in claim 15 whereinthe composition is rubber, silicone rubber, plastic, paint, a coating, acatalyst support, a glass article, a chemical mechanical polish, or aninkjet paper.
 16. A method of producing the composition as claimed inclaim 14 comprising adding the pyrogenically produced silicon dioxidepowder according to claim 1 to the composition during the manufacture ofthe composition.
 17. A method of adjusting the rheology of paint or acoating comprising adding the pyrogenically produced silicon dioxidepowder to the paint or the coating.
 18. A filler comprising thepyrogenically produced silicon dioxide powder according to claim
 1. 19.An article comprising the filler as claimed in claim 18 wherein thearticle is a rubber product, a silicone rubber product, a plasticproduct or a paper product.
 20. A method of making paper comprisingadding the pyrogenically produced silicon dioxide powder according toclaim 1 to a paper machine during the manufacture of the paper.